Some microphones are provided with microphone units and audio-signal output circuit boards that process signals from the respective microphone units and are disposed in the proximity of the respective microphone units, to reduce the overall dimensions of the microphones. In such a microphone, the audio-signal output circuit board is attached directly with solder, for example, to a circuit board provided with a field-effect transistor (FET) and installed in the microphone unit, to reduce the dimensions of the microphone section accommodating the microphone unit and the audio-signal output circuit board.
FIG. 9 is a cross-sectional side view illustrating a microphone unit included in a conventional microphone.
A microphone unit 1a includes a unit case 10a, an electroacoustic transducer accommodated in the unit case 10a, an impedance converter of the electroacoustic transducer, and a circuit board 26a. 
The unit case 10a has a shape of a hollow cylinder with a closed end. The unit case 10a is composed of pressed metal, such as aluminum. The unit case 10a has an acoustic-wave entering hole 10ah through which acoustic waves from a sound source pass. The acoustic-wave entering hole 10ah is formed on the face of the unit case 10a opposite to an opening.
The electroacoustic transducer includes a spacer 21a, a diaphragm 22a, a fixed electrode 23a, a diaphragm holder 24a, and an insulator 25a. 
The diaphragm 22a and the fixed electrode 23a face each other with the spacer 21a disposed therebetween. The diaphragm 22a and the fixed electrode 23a constitute a condenser. A layer of air having a thickness equivalent to that of the spacer 21a is formed between the diaphragm 22a and the fixed electrode 23a. 
The diaphragm 22a is a thin film composed of synthetic resin with a metal (preferably gold) film deposited on one side. The diaphragm 22a is stretched on the diaphragm holder 24a with predetermined tension.
The fixed electrode 23a is composed of metal. The fixed electrode 23a has a shape of a disk. At least one of the faces of the fixed electrode 23a, for example, the face adjacent to the diaphragm 22a, has an electret plate bonded thereto. The fixed electrode 23a and the electret plate constitute an electret board. The fixed electrode 23a is fixed to the cylindrical shaped insulator 25a composed of synthetic resin.
The disk-shaped circuit board 26a covers the opening of the unit case 10a. The circuit board 26a is fixed inside the unit case 10a by curling of the rear edge 11a of the unit case 10a. The field-effect transistor (FET) which constitutes the impedance converter is disposed on the face facing the interior of the unit case 10a among the two faces of the circuit board 26a fixed inside the unit case 10a. 
The FET includes a gate electrode E1, a drain electrode E2, and a source electrode E3. The gate electrode E1 is electrically connected to the fixed electrode 23a. The drain electrode E2 and the source electrode E3 are aligned in FIG. 9, and thus, only one of these electrodes are illustrated in FIG. 9.
The circuit board 26a has multiple holes 26ah across the thickness of the circuit board 26a (which is the horizontal direction in FIG. 9). The drain electrode E2 and the source electrode E3 pass through some of the holes 26ah. Air to be introduced to an air chamber AC disposed behind the fixed electrode 23a passes through the other holes 26ah via an acoustic resistor AR.
Solder pads (signal lands SL and ground lands GL and GL2) are disposed on the outer face facing the exterior of the unit case 10a of the circuit board 26a fixed to the unit case 10a. The drain electrode E2 and the source electrode E3 are attached to the solder pads with solder. A microphone cable (not shown) is connected to the solder pads. The ground land GL is connected to the rear edge 11a of the unit case 10a and the ground land GL2.
FIG. 10 is a cross-sectional side view illustrating components of a conventional microphone.
An audio-signal output circuit board 40a includes circuits for processing electrical signals from the microphone unit 1a. The microphone unit 1a is attached to the audio-signal output circuit board 40a with solder, for example. The microphone unit 1a and the audio-signal output circuit board 40a are accommodated in a microphone case 70a covered with a cap C.
FIG. 11 is a cross-sectional side view illustrating the ground paths in a conventional microphone.
The diaphragm holder 24a is electrically connected to a ground pattern 43a disposed on the audio-signal output circuit board 40a via the unit case 10a and the ground lands GL and GL2. The straight line connecting the ground land GL2 and the ground pattern 43a in FIG. 11 represents the electrically connected state between the ground land GL2 and the ground pattern 43a. This straight line is provided for convenience of explanation of the conventional microphone.
Schemes have been proposed to connect the unit case and the ground pattern, to prevent from generating noise due to electromagnetic waves from the connection between the microphone unit and the microphone case intruding the interior of the microphone case 70a (for example, refer to Japanese Patent Publication No. 4683996).